The invention concerns a method for automatically controlling a gas engine, in which both a fuel volume as a fraction of an air/fuel mixture is determined as a function of a set volume flow and a mixture pressure of the air/fuel mixture in the intake manifold upstream of the intake valves of the gas engine is determined as a function of a set volume flow.
Gas engines are often used as power plants for emergency generators, standby-ready units, or combined heat and power (CHP) installations. In these applications, the gas engine is operated at a combustion air ratio of, for example, 1.7, i.e., in a lean operation with air excess. Typically, the gas engine has a gas throttle valve for setting the gas fraction in the air/fuel mixture, a mixer for mixing the combustible gas with the air, a compressor as part of an exhaust gas turbocharger, a cooler and a mixture throttle valve. The mixture throttle valve serves to set the filling of the working cylinders and thus the torque of the gas engine. The filling of the working cylinders in turn is computed from the pressure of the air/fuel mixture with other parameters otherwise held constant, for example, at constant intake manifold temperature, at constant engine speed, and at constant combustion air ratio.
DE 10 2007 045 195 B3 discloses an automatic control method for a stationary gas engine with a generator, in which a speed controller uses a speed control deviation to determine a controller torque as a correcting variable. The controller torque in turn and the actual speed are used to determine a set volume flow by means of an efficiency input-output map. The set volume flow is both the input variable for controlling the gas throttle valve and the input variable for setting the mixture pressure in the intake manifold. The central element is the parallel control of the two control elements as a function of the same actuating variable, in this case, the set volume flow. The mixture pressure in the intake manifold is set via a cascade closed-loop pressure control system for the intake manifold. In this intake manifold closed-loop control system, the set intake manifold pressure represents the reference input and the measured intake manifold pressure is the controlled variable. The gas motor and the generator then constitute the controlled system. The set intake manifold pressure is computed from the set volume flow, taking into account the actual speed of the gas engine, the temperature in the intake manifold, and constants. Constant values include, for example, the combustion air ratio and a stoichiometric air requirement. The method we have been describing has been found to be effective in actual practice. However, the effect of different gas grades (volume fraction) within the same family of gases on the emission values remains critical.
DE 699 26 036 T2 also describes a method for automatically controlling a gas engine, in which a control signal for controlling the mixture throttle valve is computed from the speed control deviation by a PID controller. A correction value is determined, likewise as a function of the speed control deviation, and is then used to change the control signal for the gas throttle valve. However, the objective of the method is to suppress engine speed oscillations that develop after a change in the set engine speed.